Low-temperature lithium ion battery electrolyte as well as preparation method and application thereof

Abstract

The invention discloses a low-temperature lithium ion battery electrolyte as well as a preparation method and application thereof, and relates to the technical field of electrolytes. Through the synergistic effect of different lithium salts, the whole electrolyte has better cycling stability in a low-temperature environment; by adjusting the ratio of the carbonic ester to the short-chain carboxylic ester, the whole electrolyte system has lower viscosity and higher ionic conductivity at low temperature, so that the discharge capacity of the battery is improved; the additive is added, the synergistic effect of the additive is fully exerted, the cycle stability of the battery is improved, and the low-temperature electrolyte constructed by the method is beneficial to improving the low-temperature electrochemical performance of the battery. The preparation method is simple and convenient, the obtained low-temperature lithium ion battery electrolyte is expected to replace commercial LiPF6 to be widely applied to preparation of lithium ion batteries, and the electrochemical performance of the lithium ion batteries in a low-temperature environment is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of electrolytes, and in particular to a low-temperature lithium-ion battery electrolyte and a preparation method and application thereof. Background Art

[0002] In recent years, lithium-ion batteries (LIBs) have been widely used as electrochemical energy storage devices in portable electronic products, electric vehicles, and energy storage grids. With the vigorous development of the new energy industry, people have become more and more demanding on the scope of use and performance of LIBs. The ambient temperature will seriously affect the cycle stability, service life and safety of LIBs. It is difficult for LIBs to work in the low temperature environment of winter in low-latitude areas, and it is even more difficult to meet the requirements of energy storage and release in ultra-low temperature areas such as the polar regions and space. With the increasing demand for batteries that can work stably under low temperature conditions, improving the low temperature performance of LIBs has become a research focus in academia and industry. The low temperature performance of LIBs is closely related to factors such as the electrolyte and its compatibility with electrode materials.

[0003] Commercial LiPF6 has good electrochemical stability, does not corrode the current collector, and is easily soluble in carbonate organic solvents. － The radius is large and the association is weak at room temperature, so the LiPF6 solution has a high ionic conductivity, but it is very sensitive to trace water in the electrolyte. Hydrolysis produces HF, which may cause adverse results such as decomposition of ester solvents, dissolution of positive electrode materials and corrosion of current collectors. The existing low-temperature electrolyte of lithium-ion batteries is modified by adding additives to the traditional LiPF6-based carbonate electrolyte system, which cannot fundamentally solve the defect that LiPF6 is very easy to hydrolyze and release HF to corrode the electrode, resulting in a decrease in battery cycle performance.

[0004] In view of this, the present invention is proposed. Summary of the invention

[0005] The object of the present invention is to provide a low-temperature lithium-ion battery electrolyte and a preparation method and application thereof, so as to solve the above-mentioned technical problems.

[0006] The present invention is achieved in that:

[0007] In a first aspect, an embodiment of the present invention provides a low-temperature lithium-ion battery electrolyte, comprising a lithium salt and a mixed solvent, wherein the lithium salt comprises at least one of a first mixed lithium salt and a second mixed lithium salt; wherein the first mixed lithium salt is composed of LiBF4 and LiODFB in a molar ratio of (1-9): (9-1); and the second mixed lithium salt is composed of LiTFSI and LiODFB in a molar ratio of (5-9): (5-1);

[0008] The mixed solvent comprises carbonate and short-chain carboxylic acid ester, and the volume ratio of the short-chain carboxylic acid ester to the mixed solvent is 0.1-0.7.

[0009] In a second aspect, an embodiment of the present invention provides a method for preparing a low-temperature lithium-ion battery electrolyte as described above, the preparation method comprising:

[0010] In a glove box filled with argon, lithium salt is dissolved in a mixed solvent. After the lithium salt is completely dissolved, additives are added to prepare a low-temperature lithium-ion battery electrolyte.

[0011] In a third aspect, an embodiment of the present invention provides a use of the aforementioned low-temperature lithium-ion battery electrolyte or the aforementioned low-temperature lithium-ion battery electrolyte prepared by the aforementioned preparation method in the preparation of a lithium-ion battery.

[0012] The present invention has the following beneficial effects:

[0013] The low-temperature lithium-ion battery electrolyte provided by the present invention has better cycle stability in a low-temperature environment through the synergistic effect of different lithium salts; carbonate and short-chain carboxylic acid ester are used together as solvents of the electrolyte, and the advantages of high solubility of lithium salts due to the large dielectric constant of carbonate and low viscosity of short-chain carboxylic acid ester are combined to make the overall electrolyte system have higher ionic conductivity at low temperatures, thereby improving the discharge capacity of the battery; additives are added to give full play to their synergistic effect, construct a low-temperature electrolyte, which is suitable for use in lithium-ion battery systems at low temperatures, and improves the electrochemical properties of the electrolyte. The preparation method is simple, and the obtained low-temperature lithium-ion battery electrolyte is expected to replace commercial LiPF6 and be widely used in the preparation of lithium-ion batteries, thereby improving the electrochemical properties of lithium-ion batteries in low-temperature environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0015] Figure 1 Schematic diagram of X-ray photoelectron spectra of low-temperature lithium-ion battery electrolytes prepared in Examples 21, 27, 29, and 30;

[0016] Figure 2 Schematic diagram of cyclic voltammetry test results of low-temperature lithium-ion battery electrolytes prepared in Examples 21, 27, 29, and 30;

[0017] Figure 3Schematic diagram of the electrochemical impedance spectrum of the low-temperature lithium-ion battery electrolyte prepared in Examples 21, 27, 29, and 30. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0019] At present, the research on low-temperature electrolytes mainly focuses on the following three aspects: types of lithium salts, mixed solvents and additives.

[0020] Lithium salts have an important influence on the ionic conductivity, thermal stability and SEI film formation of the electrolyte. Compared with LiPF6, the BF bond of LiBF4 is more stable than the PF bond, resulting in higher thermal stability and lower viscosity, which makes it suitable for low-temperature electrolyte systems. In 2010, Egashira (Low-temperature performance of Li-ion cells with a LiBF 4-based electrolyte [J]. Journal of Solid State Electrochemistry, 2003, 7: 147-151. https: / / doi.org / 10.1007 / s10008-002-0300-9.) found that when LiBF4 is used as the main salt of low-temperature electrolyte, the anion radius in LiBF4 is small, which is similar to Li + The large binding energy results in low ion conductivity, and the SEI film formed on the electrode surface is unstable, which limits its scope of application. Among organic lithium salts, LiBOB and LiODFB are new boron-containing lithium salts. Compared with LiBF4, both participate in the formation of SEI film and have good compatibility with positive electrode materials. LiBOB has low solubility in chain carbonate solvents and high viscosity at low temperatures; LiODFB forms a thicker film on the electrode surface, which increases the interfacial impedance and its electrolyte exhibits poor low-temperature performance. Therefore, this type of organic lithium salt is generally not used alone. Other organic lithium salts include LiFSI and LiTFSI, both of which have high ionic conductivity and excellent film-forming properties, but because they will corrode aluminum foil, they are usually mixed with other lithium salts.

[0021] There are many types of electrolyte solvents used in LIBs, selected from at least one of carbonates, short-chain carboxylates, ethers, sulfones, nitriles and phosphates. Among them, carbonates include cyclic carbonates and chain carbonates (chain carbonates are also called linear carbonates). Cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC); chain carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC); short-chain carboxylates include propyl acetate (PA), ethyl acetate (EA), methyl propionate (MP), methyl formate (MF), methyl acetate (MA) and methyl butyrate (MB).

[0022] The ideal low-temperature electrolyte solvent should meet the following requirements as much as possible: wide liquid range, good compatibility with electrode materials and weak Li + Coordination ability and other characteristics. It is worth noting that cyclic esters have a large viscosity and their usage should be strictly controlled. Introducing a co-solvent with a low freezing point and low viscosity into the electrolyte is an effective strategy to improve ionic conductivity under low temperature conditions. The 1MLiPF6EC-DMC-DEC electrolyte proposed by Smart et al. (Development and Prospect of Low-Temperature Lithium Battery Electrolytes [J]. Energy Storage Science and Technology, 2024, 13(7): 2270. https: / / doi.org / 10.19799 / j.cnki.2095-4239.2024.0294.) combines the low melting point of DEC and the low viscosity of DMC, expanding the applicable temperature range of the electrolyte. At -20°C, the electrolyte has higher ionic conductivity and discharge capacity than EC / DEC and EC / DMC electrolytes.

[0023] The melting point and viscosity of short-chain carboxylic acid esters are lower than those of carbonate solvents, such as methyl formate (MF, melting point: -100°C), methyl acetate (MA, -98°C), methyl butyrate (MB, -84°C), ethyl acetate (EA, -84°C), ethyl propionate (EP, -73°C), etc. However, short-chain carboxylic acid ester solvents are easy to decompose and have poor effects when used alone, so short-chain carboxylic acid ester solvents are often used to replace one or more carbonate solvents in a polycarbonate mixed solvent system to form a carbonate and carboxylate mixed electrolyte system. Smart (Development and Prospect of Low-Temperature Lithium Battery Electrolytes [J]. Energy Storage Science and Technology, 2024, 13(7): 2270. https: / / doi.org / 10.19799 / j.cnki.2095-4239.2024.0294.) et al. used short-chain carboxylate solvents as co-solvents for EC-based electrolytes and found that the electrolyte containing short-chain carboxylates at -20°C had a higher discharge capacity during the initial cycle, but a poorer cycle life. The main reason is that the SEI film formed by short-chain carboxylates has a large impedance and cannot provide effective protection for the graphite negative electrode. Therefore, although changing the solvent can change the viscosity and other properties of the electrolyte, the choice of solvent also needs to focus on the solvent's effect on Li + The influence of solvent structure and SEI film stability. Improving the electrochemical performance of LIBs by electrolyte additives is considered to be an economical and effective strategy. How to effectively improve the low-temperature performance of lithium-ion batteries while maintaining the room-temperature performance of lithium-ion batteries, the present invention is now proposed.

[0024] In a first aspect, an embodiment of the present invention provides a low-temperature lithium-ion battery electrolyte, comprising a lithium salt and a mixed solvent, wherein the lithium salt comprises at least one of a first mixed lithium salt and a second mixed lithium salt; wherein the first mixed lithium salt is composed of LiBF4 and LiODFB in a molar ratio of (1-9): (9-1); and the second mixed lithium salt is composed of LiTFSI and LiODFB in a molar ratio of (5-9): (5-1);

[0025] The mixed solvent comprises carbonate and short-chain carboxylic acid ester, and the volume ratio of the short-chain carboxylic acid ester to the mixed solvent is 0.1-0.7.

[0026] It should be noted that LiBF4 in lithium salt has a small dissociation constant, low conductivity, poor room temperature performance, is not easy to form a film, and the solvent is easily embedded in the graphite layer, resulting in graphite exfoliation. However, the salt has excellent low-temperature performance, low low-temperature charge transfer impedance, and low humidity sensitivity, and is often used in low-temperature electrolyte lithium salts; LiTFSI is insensitive to water and is conducive to the formation of SEI film, but it easily corrodes the aluminum foil current collector; and the addition of LiODFB can form an interface film containing boron oxide on the surface of the current collector, inhibiting its corrosion of the current collector.

[0027] In an optional embodiment of the present invention, the first mixed lithium salt consists of LiBF4 and LiODFB in a molar ratio of (3-7): (7-3).

[0028] Specifically, the molar ratio of LiBF4 to LiODFB in the first mixed lithium salt can be selected from any one of 3:(7-3), 4:(7-3), 5:(7-3), 6:(7-3) and 7:(7-3), for example, 3:7, 4:6, 5:5, 7:3, etc. The specific ratio can be reasonably selected according to actual needs.

[0029] The molar ratio of LiTFSI to LiODFB in the second mixed lithium salt can be any one of 5:(5-1), 6:(5-1), 7:(5-1), 8:(5-1), and 9:(5-1), such as 5:5, 6:4, 7:3, 8:2, 9:1, etc. The specific ratio can be reasonably selected according to actual needs.

[0030] Prior art research shows that the use of mixed lithium salts can improve the energy density of lithium batteries and solve the problems of single salt electrolytes, such as poor thermal stability, low conductivity and lithium dendrites. By mixing lithium salts, the properties of the solution, such as ionic conductivity, lithium ion solubility and viscosity, can be changed. Mixed lithium salt electrolytes are prepared by simply mixing several lithium salts, and this method has made significant research progress in the field of next-generation lithium batteries. In addition, it was found that a dual salt electrolyte containing lithium hexafluorophosphate (LiPF6) and lithium difluorophosphate (LiDFP) was used in an ethylene carbonate / ethyl methyl carbonate (EC / EMC) mixed solvent and tested in a layered high-nickel lithium battery. The results show that this mixed salt electrolyte can effectively reduce the growth of cycle impedance and improve the overall performance of the battery.

[0031] The embodiments of the present invention optimize the ratio of different lithium salts, and the synergistic effect of the lithium salts enables the overall electrolyte to have better cycle stability in a low temperature environment.

[0032] In an optional embodiment of the present invention, the concentration of the lithium salt is 0.5 mol / L-1.0 mol / L.

[0033] In an optional embodiment of the present invention, the carbonate includes cyclic carbonate and chain carbonate; further, the volume ratio of the cyclic carbonate to the chain carbonate is 3:(1-7).

[0034] In an optional embodiment of the present invention, the ratio of the volume of the cyclic carbonate to the sum of the volumes of the linear carbonate and the short-chain carboxylic acid ester is 3:7.

[0035] Furthermore, in the embodiment of the present invention, the cyclic carbonate used is EC. EC, as an electrolyte solvent, has a high flash point and good electrochemical stability, which is conducive to the dissolution of lithium salts; the chain carbonate is EMC, which has a high dielectric constant and a low viscosity, which helps to improve the ionic conductivity of the electrolyte. Between EC and EMC, if the ratio of EC is higher, that is, more EC and less EMC, the viscosity and flash point of the electrolyte will increase, the safety and cycle life of the battery will be improved, but at the same time the ionic conductivity and low temperature performance of the battery will be reduced; if the ratio of EMC is higher, the viscosity of the electrolyte will be lower and the ionic conductivity will be higher. It is suitable for batteries that require good low temperature performance and high rate charge and discharge. For batteries that need to work in a wide temperature range, an electrolyte with a lower EC content is selected.

[0036] The short-chain carboxylic acid ester is selected from at least one of propyl acetate (PA), ethyl acetate (EA), and methyl propionate (MP). The short-chain carboxylic acid ester has a low viscosity, which can make the electrolyte system have a higher conductivity and solubility.

[0037] It should be noted that propyl acetate (PA), ethyl acetate (EA) and methyl propionate (MP) are all common organic solvents. These three compounds all belong to carboxylates, have similar chemical structures, and contain ester groups (-COO-). They are mainly used as solvents, especially in the application of lithium battery electrolytes, to improve the low-temperature performance of batteries. Among them, PA and EA are derivatives of acetic acid, but the molecular chain of PA is longer than that of EA; MP is a derivative of propionic acid, and its molecular structure is different from the former two. Ethyl acetate (EA) has significant advantages in improving low-temperature performance, but its cycle life is short, and its vapor pressure is high and it is easy to volatilize. Although the low-temperature effect of propyl acetate (PA) is slightly worse than that of ethyl EA, it is still a carboxylic acid ester with good low-temperature performance overall. The properties and uses of methyl propionate (MP) are similar to those of propyl acetate and ethyl acetate.

[0038] In an optional embodiment of the present invention, the additive is selected from any one of lithium methoxide (CH3OLi), lithium nitrate (LiNO3) and fluoroethylene carbonate (FEC), and the mass fraction of the additive is 2%-3%.

[0039] It should be noted that the additives can form a SEI film rich in inorganic components on the electrode surface, thereby improving the cycle stability of the battery at low temperatures.

[0040] In lithium metal batteries, additives are introduced to optimize battery performance, such as improving the stability of the electrolyte, improving the deposition morphology of lithium metal, and inhibiting dendrite growth. The appropriate proportion of additives can significantly improve the cycle stability and life of the battery. An inappropriate proportion may accelerate the attenuation of battery performance. Too little may not fully play its role, and the battery performance improvement is not obvious; too much will increase the viscosity of the electrolyte, reduce ion conductivity, and may even cause adverse side reactions with lithium metal, which will damage battery performance.

[0041] During the design and preparation of lithium metal batteries, the types and proportions of additives were strictly optimized through experiments, and 2% LiNO3, 2% CH3OLi, and 3% FEC were selected by mass fraction to achieve the best battery performance.

[0042] Among them, lithium methanol has good solubility and alkalinity. As an additive in lithium batteries, it can improve the crystal structure and electronic conductivity of the positive electrode material, thereby improving the electrochemical performance of the battery.

[0043] As an electrolyte additive, lithium nitrate can provide the electrolyte with higher ion conductivity. Lithium nitrate can also react with other components in the electrolyte to form a protective layer that is beneficial to battery performance, which helps to improve cycle life, high energy density and high power density, making it more durable and reliable. In addition, lithium nitrate is an attractive lithium additive that can be used to construct a Li3N-rich solid electrolyte interface (SEI) film, thereby improving the performance of lithium metal anodes.

[0044] Fluorinated ethylene carbonate is a classic additive that can be reduced on the negative electrode surface to construct a LiF-rich SEI film, which can prevent further decomposition of the electrolyte and has lower Rct and R SEI , thereby improving the low-temperature performance of the electrolyte. Specifically, FEC has a beneficial effect on the uniform and dense lithium (Li) deposition morphology, thereby improving the Coulombic efficiency (CE) of the battery chemistry during constant current cycling and extending the battery life; the electrolyte containing FEC has high conductivity and good battery performance at low temperatures, and FEC can form a stable SEI film at the negative electrode, thereby improving the low-temperature performance of the battery; FEC can also change the solvation structure and reduce Li + The coordination effect between EC molecules changes the electrochemical reaction behavior of the electrolyte on the electrode surface.

[0045] In a second aspect, an embodiment of the present invention provides a method for preparing a low-temperature lithium-ion battery electrolyte as described above, the preparation method comprising:

[0046] In a glove box filled with argon, a lithium salt is dissolved in a mixed solvent, and when the lithium salt is completely dissolved, a first low-temperature lithium-ion battery electrolyte is prepared.

[0047] Specifically, in a glove box filled with argon, a lithium salt is dissolved in a mixed solvent, stirred to dissolve all the lithium salt, and mixed evenly to obtain a first low-temperature lithium-ion battery electrolyte; the obtained low-temperature lithium-ion battery electrolyte is sealed and placed in a glove box for standby use.

[0048] In an optional embodiment of the present invention, in a glove box filled with argon, a lithium salt is dissolved in a mixed solvent, and after the lithium salt is completely dissolved, an additive is added to prepare a second low-temperature lithium-ion battery electrolyte.

[0049] In a third aspect, an embodiment of the present invention provides a use of the low-temperature lithium-ion battery electrolyte as described above or the low-temperature lithium-ion battery electrolyte prepared by the preparation method as described above in the preparation of a lithium metal battery.

[0050] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0051] Example 1

[0052] This embodiment provides a low-temperature lithium-ion battery electrolyte, and the preparation method thereof comprises the following steps:

[0053] The lithium salt raw material was prepared according to the molar ratio of LiBF4 to LiODFB of 3:7.

[0054] In a glove box filled with argon, the lithium salt raw material was dissolved in a mixed solvent of EC:EMC:PA=3:3:4 (volume ratio), stirred to dissolve all the lithium salt, and a low-temperature lithium-ion battery electrolyte was prepared, which was sealed and placed in a glove box for standby use. In the electrolyte system, the total concentration of lithium salt was 1 mol / L.

[0055] Example 2

[0056] This embodiment provides a low-temperature lithium-ion battery electrolyte, which is different from Embodiment 1 in that: in the electrolyte system, the mixed solvent is EC:EMC:PA=3:5:2.

[0057] Example 3

[0058] This embodiment provides a low-temperature lithium-ion battery electrolyte, which is different from Embodiment 1 in that: in the electrolyte system, the mixed solvent is EC:EMC:PA=3:4:3.

[0059] Example 4

[0060] This embodiment provides a low-temperature lithium-ion battery electrolyte, which is different from Embodiment 1 in that: in the electrolyte system, the mixed solvent is EC:EMC:PA=3:6:1.

[0061] Example 5

[0062] This embodiment provides a low-temperature lithium-ion battery electrolyte, which is different from Embodiment 1 in that: in the electrolyte system, the mixed solvent is EC:EMC:PA=3:2:5.

[0063] Example 6

[0064] This embodiment provides a low-temperature lithium-ion battery electrolyte, which is different from Embodiment 1 in that: in the electrolyte system, the mixed solvent is EC:EMC:EA=3:3:4.

[0065] Example 7

[0066] This embodiment provides a low-temperature lithium-ion battery electrolyte, which is different from Embodiment 1 in that: in the electrolyte system, the mixed solvent is EC:EMC:MP=3:3:4.

[0067] Example 8

[0068] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 1 in that the lithium salt raw material is prepared according to a molar ratio of LiBF4 to LiODFB of 5:5.

[0069] In a glove box filled with argon, the lithium salt raw material was dissolved in a mixed solvent of EC:EMC:PA=3:3:4 (volume ratio), and stirred to dissolve all the lithium salt to obtain a low-temperature lithium-ion battery electrolyte, which was sealed and placed in the glove box for standby use.

[0070] Example 9

[0071] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 8 in that: in the electrolyte system, the mixed solvent is EC:EMC:EA=3:3:4.

[0072] Example 10

[0073] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 8 in that: in the electrolyte system, the mixed solvent is EC:EMC:MP=3:3:4.

[0074] Embodiment 11

[0075] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 8 in that the lithium salt raw material is prepared according to a molar ratio of LiBF4 to LiODFB of 7:3.

[0076] Example 12

[0077] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 9 in that the lithium salt raw material is prepared according to a molar ratio of LiBF4 to LiODFB of 7:3.

[0078] Embodiment 13

[0079] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 10 in that the lithium salt raw material is prepared according to a molar ratio of LiBF4 to LiODFB of 7:3.

[0080] This embodiment 14

[0081] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 8 in that the lithium salt raw material is prepared according to a molar ratio of LiTFSI to LiODFB of 9:1.

[0082] Embodiment 15

[0083] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 9 in that the lithium salt raw material is prepared according to a molar ratio of LiTFSI to LiODFB of 9:1.

[0084] Example 16

[0085] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 10 in that the lithium salt raw material is prepared according to a molar ratio of LiTFSI to LiODFB of 9:1.

[0086] Embodiment 17

[0087] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 8 in that the lithium salt raw material is prepared according to a molar ratio of LiTFSI to LiODFB of 8:2.

[0088] Embodiment 18

[0089] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 9 in that the lithium salt raw material is prepared according to a molar ratio of LiTFSI to LiODFB of 8:2.

[0090] Embodiment 19

[0091] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 10 in that the lithium salt raw material is prepared according to a molar ratio of LiTFSI to LiODFB of 8:2.

[0092] Embodiment 20

[0093] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 1 in that the lithium salt raw material is prepared according to a molar ratio of LiBF4 to LiODFB of 5:5.

[0094] In a glove box filled with argon, the lithium salt raw material was dissolved in a mixed solvent of EC:EMC:PA=3:3:4 (volume ratio), and stirred to dissolve all the lithium salt; 3% of FEC additive was added, mixed evenly, and a low-temperature lithium-ion battery electrolyte was obtained, which was sealed and placed in a glove box for standby use. In the electrolyte system, the total concentration of lithium salt was 1 mol / L.

[0095] Embodiment 21

[0096] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 20 in that: in the electrolyte system, the mixed solvent is EC:EMC:EA=3:3:4.

[0097] Embodiment 22

[0098] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 20 in that: in the electrolyte system, the mixed solvent is EC:EMC:MP=3:3:4.

[0099] Embodiment 23

[0100] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 1 in that the lithium salt raw material is prepared according to a molar ratio of LiTFSI to LiODFB of 9:1.

[0101] In a glove box filled with argon, the lithium salt raw material was dissolved in a mixed solvent of EC:EMC:PA=3:3:4 (volume ratio), and stirred to dissolve all the lithium salt; the lithium salt was prepared with a molar ratio of LiTFSI to LiODFB of 9:1, and a mixed solvent of EC:EMC:PA=3:3:4 (volume ratio) and a CH3OLi additive with a mass fraction of 2% were added, and mixed evenly to obtain a low-temperature lithium-ion battery electrolyte, which was sealed and placed in a glove box for standby use. Among them, in the electrolyte system, the total concentration of lithium salt is 1 mol / L.

[0102] Embodiment 24

[0103] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 23 in that: in the electrolyte system, the mixed solvent is EC:EMC:EA=3:3:4.

[0104] Embodiment 25

[0105] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 23 in that: in the electrolyte system, the mixed solvent is EC:EMC:MP=3:3:4.

[0106] Embodiment 26

[0107] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 23 in that:

[0108] In a glove box filled with argon, the lithium salt raw material was dissolved in a mixed solvent of EC:EMC:PA=3:3:4 (volume ratio), and stirred to dissolve the lithium salt completely; a lithium salt prepared with a molar ratio of LiTFSI to LiODFB of 9:1 and a mixed solvent of EC:EMC:PA=3:3:4 (volume ratio) was added with a 2% LiNO3 additive by mass, and mixed evenly to obtain a low-temperature lithium-ion battery electrolyte, which was sealed and placed in a glove box for standby use. Among them, the total concentration of lithium salt in the electrolyte system was 1 mol / L.

[0109] Embodiment 27

[0110] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 26 in that: in the electrolyte system, the mixed solvent is EC:EMC:EA=3:3:4.

[0111] Embodiment 28

[0112] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 26 in that: in the electrolyte system, the mixed solvent is EC:EMC:MP=3:3:4.

[0113] Embodiment 29

[0114] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 27 in that: in the electrolyte system, the total concentration of lithium salt is 1.3 mol / L.

[0115] Embodiment 30

[0116] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 27 in that: in the electrolyte system, the total concentration of lithium salt is 0.7 mol / L.

[0117] Embodiment 31

[0118] This embodiment provides a low-temperature lithium-ion battery electrolyte, which differs from Embodiment 27 in that: in the electrolyte system, the total concentration of lithium salt is 0.5 mol / L.

[0119] Comparative Example 1

[0120] This comparative example provides a low-temperature lithium-ion battery electrolyte, which is different from Example 1 in that: in the electrolyte system, the mixed solvent is EC:EMC=3:7 (commercial solvent system).

[0121] Comparative Example 2

[0122] This comparative example provides a low-temperature lithium-ion battery electrolyte, which is different from Example 8 in that: in the electrolyte system, the mixed solvent is EC:EMC=3:7.

[0123] Comparative Example 3

[0124] This comparative example provides a low-temperature lithium-ion battery electrolyte, which differs from Example 11 in that in the electrolyte system, the mixed solvent is EC:EMC=3:7.

[0125] Comparative Example 4

[0126] This comparative example provides a low-temperature lithium-ion battery electrolyte, which differs from Example 14 in that in the electrolyte system, the mixed solvent is EC:EMC=3:7.

[0127] Comparative Example 5

[0128] This comparative example provides a low-temperature lithium-ion battery electrolyte, which differs from Example 17 in that in the electrolyte system, the mixed solvent is EC:EMC=3:7.

[0129] Comparative Example 6

[0130] This comparative example provides a low-temperature lithium-ion battery electrolyte, which is different from Example 1 in that: in the electrolyte system, the mixed solvent is EC:EMC:MA=3:3:4.

[0131] Test Example 1

[0132] In this test example, the low-temperature lithium-ion battery electrolyte prepared in Examples 1-31 and Comparative Examples 1-6 was assembled into batteries and the cycle performance was tested. The specific test method is as follows: lithium cobalt oxide is used as the positive electrode, lithium sheet is used as the negative electrode, aluminum foil is used as the current collector, and Celgard 2400 is used as the diaphragm. The button half-cell is assembled in the glove box and tested after standing for 12 hours. The battery is activated by charging and discharging at 1 / 10C 2.7V to 4.2V at room temperature. Some embodiments can work normally at 0.1C charging and discharging at -40℃. The cycle test results are shown in Table 1.

[0133] Table 1 Performance test results of low temperature lithium ion battery electrolyte

[0134]

[0135]

[0136]

[0137] It can be seen from Table 1 that the data of Examples 1-31 and Comparative Examples 1-6 show that the combination of three short-chain carboxylates PA, EA and MP with EC and EMC respectively in the present invention can mostly improve the low-temperature discharge specific capacity and cycle stability of the battery. This is because the short-chain carboxylates have lower viscosity at low temperatures, especially the addition of EA can make the discharge specific capacity of the battery at low temperatures closer to the theoretical value, such as Examples 9, 18, 21, and 27. It is worth mentioning that the low-temperature performance of the battery is jointly determined by the ionic conductivity and the properties of the solid electrolyte interface film. Therefore, under the optimal solvent system, the proportion of mixed lithium salts and the types of additives were explored. It was found that the electrolyte systems containing the additives FEC and LiNO3 showed better low-temperature electrochemical performance. When the total concentration of lithium salts was 0.7M, LiTFSI 0.9 / LiODFB 0.1 -EC / EMC / EA (3:3:4) + 2% LiNO3 electrolyte system still has the best low temperature performance at -40°C. From Examples 27, 29-31, it can be determined that the optimal lithium salt concentration is 0.5mol / L-1.0mol / L. By comparing Comparative Examples 1-5 with Examples 1, 8, 11, 14 and 17, the optimal ratio of the first mixed lithium salt LiBF4 and LiODFB is (3-7): (7-3); the optimal ratio of the second mixed lithium salt LiTFSI and LiODFB is (5-9): (5-1). At the same time, Comparative Example 6 shows that the low temperature performance of the battery can also be improved by using short-chain carboxylic acid ester MA with low viscosity, low freezing point and high dielectric constant.

[0138] Test Example 2

[0139] In this test example, the chemical properties and composition of the electrodes of the batteries assembled with the low-temperature lithium ion electrolyte prepared in the four groups of Examples 21, 27, 29, and 30 having the best performance in discharge specific capacity and cycle stability after cycling were analyzed by X-ray photoelectron spectroscopy, and the instrument model was Thermo Scientific Escalab 250Xi + The experimental results were processed by Avantage software. Figure 1 .

[0140] from Figure 1 It can be seen that the electrolyte prepared in Example 21 forms less inorganic compounds on the electrode surface. However, the electrolyte containing lithium nitrate additive in Example 27 can form an inorganic-rich interface film on the electrode surface. This is because NO3 -Ions have strong polarity and can participate in the solvation of lithium ions and weaken the interaction between lithium ions and solvents. After the low-concentration electrolyte prepared in Example 30 is circulated, the electrode surface contains more inorganic components and fewer organic components. This may be due to the reduction in the activation energy of lithium ion desolvation caused by low concentration, which is specifically manifested in the reduction of the charge transfer impedance of the battery.

[0141] Test Example 3

[0142] In this test example, the battery assembled with the electrolyte prepared in Examples 21, 27, 29, and 30 was subjected to cyclic voltammetry test. The instrument model was Chenhua CHI660B electrochemical workstation. -20°C, voltage range: (2.5V-5.5V), scan rate: 5×10 -4 V / s. Related results are shown in Figure 2 .

[0143] from Figure 2 It can be seen that the electrolyte prepared in Example 30 has a smaller ΔEp, a sharper peak in the CV curve, and a smaller polarization degree, while the electrolyte in Example 21 without additives has a larger ΔEp in the CV curve, a flatter peak, and poor kinetic performance. This indicates that the addition of an appropriate amount of LiNO3 can further reduce the polarization of the battery and improve the low-temperature performance of the battery.

[0144] Test Example 4

[0145] In this test example, the low-temperature lithium-ion battery electrolyte prepared in Examples 21, 27, 29, and 30 was tested using an electrochemical impedance spectrometer. The instrument model was Chenhua CHI660B electrochemical workstation, frequency: 10 -2 Hz-10 5 Hz; Amplitude: 5mV. Related results are shown in Figure 3 , wherein (a) represents Example 21, (b) represents Example 27, (c) represents Example 29, and (d) represents Example 30.

[0146] from Figure 3 It can be seen that the impedance of the battery is larger when it is cycled at -20℃. This is because the interface film becomes thicker under low temperature conditions, and the Li + The migration rate is reduced and the reaction rate is slowed down. In the absence of additives, the total impedance of the electrolyte of Example 21 is the largest at each stage (10, 50, and 100 cycles), while that of the electrolyte containing additives is greatly reduced. This may be due to the addition of lithium nitrate, which reduces the difficulty of lithium ion desolvation and forms a uniform and dense film layer on the electrode surface.

[0147] In summary, the low-temperature lithium-ion battery electrolyte provided by the present invention adopts a mixed lithium salt, which combines the advantages of two lithium salts, and selects a mixed solvent and an additive, and improves the electrochemical performance of the lithium-ion battery in a low-temperature environment through its synergistic effect. The prepared lithium-ion battery can work normally in an environment of -40°C to 10°C, and is expected to replace commercial LiPF6 and be widely used in the preparation of lithium-ion batteries.

[0148] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A low-temperature lithium-ion battery electrolyte, comprising a lithium salt and a mixed solvent, characterized in that: The lithium salt comprises at least one of a first mixed lithium salt and a second mixed lithium salt; wherein the first mixed lithium salt is composed of LiBF4 and LiODFB in a molar ratio of (1-9): (9-1); and the second mixed lithium salt is composed of LiTFSI and LiODFB in a molar ratio of (5-9): (5-1); The mixed solvent comprises carbonate and short-chain carboxylic acid ester, and the volume ratio of the short-chain carboxylic acid ester to the mixed solvent is 0.1-0.

7.

2. The low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that: The first mixed lithium salt is composed of LiBF4 and LiODFB in a molar ratio of (3-7): (7-3).

3. The low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that: The concentration of the lithium salt is 0.5 mol / L-1.0 mol / L.

4. The low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that: The carbonates include cyclic carbonates and chain carbonates; Preferably, the ratio of the volume of the cyclic carbonate to the volume of the linear carbonate is 3:(1-7).

5. The low-temperature lithium-ion battery electrolyte according to claim 4, characterized in that: The ratio of the volume of the cyclic carbonate to the sum of the volumes of the chain carbonate and the short-chain carboxylic acid ester is 3:

7.

6. The low-temperature lithium-ion battery electrolyte according to claim 5, characterized in that: The short-chain carboxylic acid ester is selected from at least one of propyl acetate (PA), ethyl acetate (EA), methyl propionate (MP), methyl formate (MF), methyl acetate (MA) and methyl butyrate (MB).

7. The low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that: The low-temperature lithium-ion battery electrolyte further includes an additive, wherein the mass fraction of the additive is 2%-3%; Preferably, the additive is selected from any one of lithium methoxide, lithium nitrate and fluoroethylene carbonate.

8. A method for preparing a low-temperature lithium-ion battery electrolyte according to any one of claims 1 to 7, characterized in that: The preparation method comprises: In a glove box filled with argon, a lithium salt is dissolved in a mixed solvent, and when the lithium salt is completely dissolved, a first low-temperature lithium-ion battery electrolyte is prepared.

9. The preparation method according to claim 8, characterized in that: The preparation method comprises: dissolving a lithium salt in a mixed solvent in a glove box filled with argon gas, adding an additive after the lithium salt is completely dissolved, and obtaining a second low-temperature lithium-ion battery electrolyte.

10. Use of the low-temperature lithium-ion battery electrolyte according to any one of claims 1 to 7 or the low-temperature lithium-ion battery electrolyte prepared by the preparation method according to any one of claims 8 to 9 in preparing a lithium-ion battery.

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